An electroluminescent (EL) device is a self-light-emitting device which has advantages in that it provides a wider viewing angle, a greater contrast ratio, and a faster response time. An organic EL device was first developed by Eastman Kodak, by using small aromatic diamine molecules and aluminum complexes as materials to form a light-emitting layer [Appl. Phys. Lett. 51, 913, 1987].
The most important factor determining luminous efficiency in the organic EL device is light-emitting materials. Until now, fluorescent materials have been widely used as light-emitting material. However, in view of electroluminescent mechanisms, since phosphorescent materials theoretically enhance luminous efficiency by four (4) times compared to fluorescent materials, phosphorescent light-emitting materials have been widely researched. Iridium(III) complexes have been widely known as phosphorescent materials, including bis(2-(2′-benzothienyl)-pyridinato-N,C-3′)iridium(acetylacetonate) ((acac)Ir(btp)2), tris(2-phenylpyridine)iridium (Ir(ppy)3) and bis(4,6-difluorophenylpyridinato-N,C2)picolinate iridium (Firpic) as red-, green- and blue-emitting materials, respectively.
At present, 4,4′-N,N′-dicarbazol-biphenyl (CBP) is the most widely known host material for phosphorescent materials. Recently, Pioneer (Japan) et al., developed a high performance organic EL device using bathocuproine (BCP) and aluminum(III) bis(2-methyl-8-quinolinate)(4-phenylphenolate) (BAlq) etc., as host materials, which were known as hole blocking materials.
Although these materials provide good light-emitting characteristics, they have the following disadvantages: (1) Due to their low glass transition temperature and poor thermal stability, their degradation may occur during a high-temperature deposition process in a vacuum, which results in poor lifespan. (2) The power efficiency of the organic EL device is given by [(π/voltage)×current efficiency], and the power efficiency is inversely proportional to the voltage. Although the organic EL device comprising phosphorescent host materials provides higher current efficiency (cd/A) than one comprising fluorescent materials, a significantly high driving voltage is necessary. Thus, there is no merit in terms of power efficiency (Im/W). (3) Furthermore, the operational lifespan of the organic EL device is short, and luminous efficiency is still required to be improved. Accordingly, in order to embody the advantageous properties of the organic EL device, it is important to suitably select the materials that are comprised in the organic layer of the organic EL device, especially the host or dopant materials that are comprised in the light-emitting materials.
Japanese Patent Application Laying-Open No. 2014-160813 discloses an organic electroluminescent device comprising a nitrogen-containing heteroaryl compound formed by condensing a pyrrole ring, an aromatic aryl ring, and a 7-membered aryl ring, as host/dopant materials. However, it does not specifically disclose a compound in which a carbazole and an indole are fused with a 7-membered ring.
Korean Patent Application Laying-Open No. 10-2015-077220 discloses a compound in which an amine radical is fused with dibenzo residue including a carbazole, fluorene, or 5-membered hetero ring to form a ring. However, it does not specifically disclose a compound wherein a parent nucleus formed by a fusion of a carbazole, an indole, and a 7-membered ring, is connected, directly or via a linker of arylene or heteroarylene, to a 10-membered nitrogen-containing heteroaryl.